The several common protocols for local area networks (LANs) include CSMA/CD (Carrier Sense Multiple Access with Collision Detection), token bus, and token ring. CSMA/CD is sometimes generically, but incorrectly, referred to as Ethernet, which is a product of the XEROX corporation using the protocol. I.E.E.E. has promulgated standards for these protocols, collectively known as IEEE 802, or also known as ISO 8802. IEEE 802.3 covers one-persistent CSMA/CD LAN; IEEE 802.4 and 802.5 cover token ring and token bus, respectively. These standards differ at the physical layer but are compatible at the data link layer in the seven layer OSI (Open Systems Interconnection) reference model.
CSMA/CD, token bus, and token ring are similar in the sense that they are all packet or frame based systems in which inter-node communications are broadcast over a shared transmission medium. In CSMA/CD, a node wishing to transmit over the network cabling listens to ensure that the network is idle, i.e., no other node is currently transmitting. When the network is idle, the node may begin transmission. Due to the physical extent of the cable, however, the simultaneous transmission of two or more nodes may occur. This gives rise to what is termed a collision. To compensate for this eventuality, each node also listens while it transmits. In some cases, the average voltage during the transmission will be different if a collision is occurring on the network. In other cases, a jamming signal will be generated by a network hub unit. Each node should terminate their respective transmissions during a collision and generate a jamming signal for a predetermined period. The nodes then individually wait for a random time interval before seeking to retransmit.
Token bus and ring architectures mediate access to the network cabling by passing an abstraction known as a token between nodes. A node must wait until it receives the token before it may transmit. If the node receives the token but does not wish to transmit or once it has finished its transmission, it simply passes the token to the next node, by signaling that node. Under this system, collisions should never occur. Thus, there is no requirement that the nodes listen during their transmissions as required by CSMA/CD.
Different protocols can be used in networks that have larger physical extent such as metropolitan area networks (MANs) and wide area networks (WANs). MAN protocols tend to be similar to the LAN protocols. WANs typically have comparatively low data rates. Also, lower reliability increases the need for more error checking. WAN protocols are selected to compensate for these differences.
Other technologies are also emerging. Asynchronous transfer mode, more commonly known as ATM, is specially designed for inter-network communications. It relies on fixed sized packets which makes the protocol suboptimal for most, but compatible with virtually all, applications, but this compromise increases the speed at which the packets can be routed. Optical fiber based systems are becoming more common such as the fiber distributed data interface (FDDI).
In each protocol, the nodes must comply with the relevant rules that dictate the timing of transmissions to fairly allocate access to the network's transmission bandwidth. Proper operation also dictates the format for the transmitted data. Packets must usually include a preamble to synchronize data decoding, comply with an error detection/correction scheme, and meet requirements for maximum and minimum lengths. There are a few techniques or devices that enable a network administrator to detect the violation of these rules, enabling diagnosis and location of the problems in the networks.
Protocol analyzers and remote monitoring (RMon) probes are commercially available devices that decode properly formatted digital transmissions on LANs, or similar networks. The devices function as passive network nodes that acquire packets and detect the cable voltages that are indicative of collisions. The origin, destination, and number of packets can be determined by reference to the packet's headers and bandwidth utilization statistics accumulated for analysis. The number and frequency of collisions can also be monitored.
FIG. 1 illustrates the architecture for the network interface portion 1410 of a protocol analyzer or RMon probe, which incidently is similar to any other network interface chip for a node in a CSMA/CD-type network. The interface comprises a phase-locked loop 1420 that uses each packet's preamble to synchronize to the source node. A decoder 1430 then extracts the destination address DA, source address SA, and data from the packet and performs error checking in response to a cyclic redundancy check CRC data contained in the frame check sequence (FCS) to ensure the packet 1440 is valid. On the assumption that it is, the decoder 1430 sends out only the destination address DA, source address SA, and data on the output line 1450. Simultaneously, a d.c. voltage threshold detector 1460 monitors the average voltage on the input line. In the example of 10Base(2) and (5), it will indicate a collision if the magnitude of the input voltage is more negative than −1.6 Volts. This occurs because the simultaneous transmission from two or more sources are additive on the network cable. When a collision is detected, the threshold detector generates the signal on a collision sense line 1470 and also disables the decoder 1430.
Two packets 1440 and a noise signal 1480 represent successive inputs to the network interface 1410. The analyzer can only interpret properly formatted packets, however. Noise 1480 is not detectable by the device. Moreover, if the noise exceeds the −1.6 Volt threshold of the detector 1460, the network interface 1410 may actually indicate the presence of a collision, but the source will not have been from typical network traffic.
In many cases, the protocol analyzers or RMon probes will not properly capture even valid packets on the network. If the gap between packets is less than 9.6 microseconds known as the inter-frame gap (IFG), the chip will usually miss the second in-time packet. Further, transmissions experiencing excessive attenuation or originating from a bad transmitter can result in collisions that are below the collision threshold. As a result, the analyzer will still attempt to decode the transmissions since the decoder will not be disabled. These devices can also saturate when a series of packet transmissions occur in quick succession.
Some of the shortcomings in the protocol analyzer and RMon probes are compensated by techniques that enable the analog analysis of the network transmission media. The most common one is called time domain reflectometry (TDR). According to this technique, a pulse of a known shape is injected into the cabling of the network. As the pulse propagates down the cable and hits electrical “obstacles,” or changes in the cable's characteristic impedance, an echo is generated that travels back to the point of injection. The existence of the echo can indicate cable breaks, frayed cables, bad taps, loose connections or poorly matched terminations. The time interval between the initial transmission of the pulse and the receipt of the echo is a function of a distance to the source of the echo. In fact, by carefully timing this interval, the source of the echo can be located with surprising accuracy.
TDR analysis is typically used by installers to ensure that the newly laid wiring does not have any gross faults. The TDR signal is injected into the wiring while the network is non-operational to validate the transmission media. If a network is already installed, the network is first turned off so that TDR analysis can be performed. In a star topology network, the manager will typically check each link between the hub and host, marking any suspect wires. In bus topologies, the TDR signal is generated on the main trunk. In either case, reflections indicate breaks or defects in the network cables.